1. Field of the Invention
The present invention relates to solid state electrochemical devices and, more particularly, to solid state electrochemical devices in which the electrolyte is a polymeric network, such as one interpenetrated by an ionically conducting liquid phase.
2. State of the Art
Traditional batteries, employing aqueous solutions as the electrolytes, have given way to electrochemical devices, such as batteries and capacitors, which have a solid electrolyte. Unlike their aqueous electrolyte counterparts, the solid electrolyte devices offer the advantages of thermal stability, absence of corrosion of the electrodes, and the ability to be manufactured in thin layers.
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art. These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte, i.e., liquid batteries, including improved safety features.
The solid electrolyte is interposed between the cathode and the anode. To date, the solid electrolytes have been constructed from either inorganic or organic matrices including a suitable inorganic ion salt. The inorganic matrix may be non-polymeric [e.g, .beta.-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazine) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, ethylene oxide, propylene oxide, ethyleneimine, epichlorohydrin, ethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 =CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix.
Preferably, the solid polymeric matrix is an organic matrix derived from a solid matrix-forming monomer and/or from partial polymers of a solid matrix-forming monomer.
One problem which research efforts have attempted to overcome in the design of solid state electrochemical cells from a solid matrix is the poor conductivity of polymeric electrolytes at room temperature and below. In many cases, the cells which have been designed to date are used at elevated temperatures due to the low conductivity of the electrolyte at ambient temperatures.
In addition to providing a high ionic conductivity, it is important that a solid electrolyte also provide good mechanical strength. Unfortunately, there is a tendency for these two properties to oppose one another. Attempts to increase conductivity usually involve a reduction in molecular weight and result in fluid or mechanically weak films. Techniques, such as crosslinking, increase film strength but generally result in a loss in conductivity.
The problem of striking a suitable balance between these two mutually exclusive properties has been solved to some extent by providing a solid electrolyte which is a two phase interpenetrated network of a mechanically supporting phase of a continuous network of a crosslinked polymer and an ionic conducting phase comprising a metal salt and a complexing liquid polymer such as liquid polyethylene oxide complexed with a lithium salt, as set forth in U.S. Pat. No. 4,654,279. As explained therein, the mechanically supporting phase forms a matrix which supports an interpenetrating ionically conducting liquid polymer phase which provides continuous paths of high conductivity throughout the matrix. Representative examples of the mechanically supporting phase described in U.S. Pat. No. 4,654,279 are epoxies, polyurethanes, polyacrylates, polymethacrylates, polystyrenes and polyacrylonitriles.
The solvent (plasticizer) is typically added to the matrix in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte are recognized by those skilled in the art to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
These electrolyte solvents are solvents capable of complexing the electrolytes so as to render the salt mixture ionically conductive. In this regard, the electrolyte solvent can be a single solvent or a mixture of two or more solvents selected to enhance the complexing and thus improve the electrochemical properties of the resulting cell.
Suitable electrolyte solvents known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like. Other suitable solvents, as disclosed in U.S. Pat. No. 5,289,143 and Ser. No. 07/918,508 (Attorney Docket No. 028574-046) of common assignee which are herein incorporated by reference in their entirety, include mixtures of organic carbonates, e.g., propylene carbonate, and triglyme.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
The development of the solid electrolyte including the two phase interpenetrated network of a mechanically supporting phase of a continuous network of a crosslinked polymer and an ionic conducting phase comprising a metal salt of a complexing liquid polymer overcame to a significant extent the problem of striking a balance between good mechanical strength on the one hand and good conductivity on the other hand.
One particularly preferred solid electrolyte battery, including a crosslinked polymeric phase and an ionic conducting phase, employs lithium as the anode. In particular, lithium has been of interest due to its low density and highly electropositive nature.
A typical cell will incorporate, for example, a lithium or lithium based anode and a cathode containing a vanadium oxide compounds, e.g., V.sub.6 O.sub.13, as the active material. The lithium anode may be a metal foil. The electrolyte layer comprises a polymeric matrix, lithium salt, and an electrolyte solvent comprising propylene carbonate. The cathode structure consists of a composite material containing the active cathode material, e.g., V.sub.6 O.sub.13, a polymeric electrolyte material, and carbon, e.g., in the form of acetylene black. These batteries have been found to be beneficial in terms of ease of construction, ruggedness, interfacial properties, open circuit voltage, energy density, and rechargeability.
The solid, solvent-containing electrolyte is typically formed in one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the solid matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the electrolytic solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
With respect to the first of the above two techniques for forming the solid electrolyte, there are obviously required two separate solvents, the first of which is a volatile one which evaporates before or during the crosslinking step and the second of which remains in the solid electrolyte after crosslinking to provide the solvent phase for the ionic salt. The second technique employs only one solvent which serves both as the agent for solvating the prepolymer before and during crosslinking and as the solvent for the ionic conducting phase after crosslinking has been completed.
Quite clearly, the second technique for forming a solid technique has the advantage, as compared to the first technique, of only requiring a single solvent for the dual function of solvating the prepolymer before crosslinking and solvating the ionic conducting phase after crosslinking. To achieve both of these functions simultaneously, the art has had to resort to the use of a solvent having a relatively high boiling point, i.e., above 100.degree. C., in order that enough solvent remain after the crosslinking to solvate the ionically conductive phase in the crosslinked polymeric electrolyte. In particular, as the solvated electrolyte precursors are exposed to the open air both when applied to an underlying substrate and when crosslinked as by electron beam radiation, it was found that only a solvent of relatively high boiling point would remain in sufficient quantities after crosslinking so as to solvate the ionically conducting phase.
The use of such high boiling point solvents has prevented undesirable evaporation of solvent from the monomer/partial polymer prior to crosslinking. Nonetheless, they have been found to give rise at the same time to a distinct disadvantage in that such high boiling solvents affect the performance of the electrolytic device, especially low temperatures, e.g., below ambient. In particular, it has been found that the higher boiling solvents, as opposed to their more volatile lower boiling counterparts, become quite viscous and thus interfere with the distribution of the ionically conductive phase in the solid electrolyte.
Thus, the only manner in which the art to date has been able to manufacture solid state electrochemical devices employing a dual use solvent (where the solvent serves both as the vehicle for dissolving the electrolyte precursor components and as the solvent for the ionically conductive phase) has been to solely employ high boiling point solvents which compromise the low temperature performance of the electrochemical device.